Communication systems that utilize coded communication signals are known in the art. One such system is a direct sequence code division multiple access (DS-CDMA) cellular communication system, such as set forth in the Telecommunications Industry Association Interim Standard 95A (TIA/IEIA IS-95A) herein after referred to as IS-95A. In accordance with IS-95A, the coded communication signals used in the DS-CDMA system comprise signals that are transmitted in a common 1.25 MHz bandwidth, hence, spread-spectrum, to base sites of the system from communication units, such as mobile or portable radiotelephones, that are communicating in the coverage areas of the base sites. Each 1.25 MHz bandwidth portion of the radio-frequency (RF) spectrum is commonly referred to as a carrier frequency, capable of conveying multiple sync, paging and digital voice channels associated with a CDMA communication signal.
In a cellular communication system, a pair of communication links is established between a mobile station, or subscriber, and a source base transceiver station. As a mobile station moves out of range of the source base transceiver station, the signal quality degrades until one of the pair of the communication links is ultimately broken, or the call "dropped". To avoid loss of the communication links resulting from a dropped call, the communication links are shifted from the source base transceiver station to a target base transceiver station, or from a source sector to a target sector within the source base transceiver station coverage area. This process of making the shift is commonly referred to in the cellular communication area as a handoff process. A handoff may occur during a call in progress (e.g. from a traffic channel to a traffic channel), or during the initial signaling during call set-up.
Handoffs are generally classified into three types; a soft handoff, a softer handoff and a hard handoff. A soft handoff occurs when a mobile communication signal is transferred from a source base transceiver station (BTS) to a target BTS, the BTSs serving different cell coverage areas. The transfer occurs while the mobile station is in communication with both the source and target BTSs. Similarly, a softer handoff occurs when a mobile communication signal is transferred from a source sector to a target sector, both sectors associated with the same base transceiver station. The transfer occurs while the mobile station is in communication with both the source and target sector. During a soft and softer handoff, the mobile communication signal is supported simultaneously by both the source and target until the transfer to the target is complete. A hard handoff may occur when a mobile station is directed to re-tune to a new carrier frequency, and/or the control of resources supporting the mobile communication signal is transferred from a source base station controller to a target CBSC.
In digital cellular systems such as time division multiple access (TDMA) and code division multiple access (COMA) systems, handoffs are generally initiated by a mobile station and are commonly referred to as mobile assisted handoffs (MAHO). CDMA MAHOs are generally initiated by the mobile station based upon measurements of local pilot signals emanating from neighboring BTS sectors and/or BTSs, wherein each pilot signal includes a signal strength measurement Ec/Io (Energy per chip divided by total interference), and an associated short code PN time offset. Each short code PN time offset corresponds to a specific sector in BTS coverage area and is generated via time shifting (also referred to as phase shifting) a predetermined pseudorandom noise sequence from an absolute time offset.
In IS-95 and J-STD-008 CDMA systems, one common pseudorandom noise sequence is utilized. The pseudorandom noise sequence with a bit rate of 1,228,800 bits (chips) per second is time shifted, as measured by chips, to provide an identifiable digital sequence in the communication signal. For example, a PN offset of 1 corresponds to the pseudorandom noise sequence, shifted 64 chips from an absolute time offset, while a PN offset of 2 corresponds to the pseudorandom noise sequence, shifted 128 chips from an absolute time offset. Accordingly, IS-95 requires extremely accurate time synchronization, for example synchronization to within +/-3 usec., at each base site in order to reliably handoff a mobile station communication signal. As a result, base stations typically receive their absolute system time (a.k.a. timing synchronization) via a global positioning satellite (GPS), although other accurate central timing sources such as LORAN-C may be used.
For a variety of reasons, some base stations do not have access to system timing synchronization. These reasons may include GPS outages, customer adversity to using GPS timing, as well as the physical location of the base station. For example, if no GPS is used in a CDMA system, it would be desirable to time synchronize all BTSs to one master BTS. In another example, a base station located in a subway tunnel, without benefit of system timing synchronization provided by line-of-sight access to GPS, is highly unlikely to provide handoff capability for a mobile station communication signal. As a result, in order to provide handoff capability for a mobile station communication signal, supplementary cabling etc. expenses associated with providing access to a GPS receiver, are incurred.
In addition, the use of home base cordless phone systems utilizing DS-CDMA technology pose another type of synchronization problem. Because they are not centrally synchronized via sophisticated synchronization sources such as GPS, it is possible for the short code PN time offsets of adjacent home base transceiver stations to "drift" until they align with each other. Upon alignment, signals transmitted from one home base transceiver station will destructively interfere with the other home base transceiver station and vise versa. Unfortunately, this destructive interference will render one or both of the home base transceiver stations unusable because its associated mobile station will not be able to differentiate between it and the adjacent home base transceiver station.
Therefore, a need exists for a method and apparatus for providing a time adjustment to a wireless communication system.